New hydraulic design – faster through the curve

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Scientists of the Finish Tampere Technical University have dealt with the possible applications of digital hydraulics for years. After stationary applications, they have also tested the mobile use and predicted clear advantages with regard to robustness and availability of tilting systems in railway technology. As scientific company, the Finish National Railway (VR) supported them in the validation of these advantages, using a digital-hydraulic solution as retrofit kit for the existing Pendolino fleet as example.

With about 10,000 students, Tampere Technical University (TUT) is the country’s second largest TU. About 1,800 employees do teaching and research work here. The university is closely linked to the industry and reaches very high values in the international comparison. In the Times Higher Education (THE) ranking, the TUT ranks 11th, worldwide, in the “Share of publications based on direct cooperation with industrial companies” category. One focus is digital hydraulics and their different possible applications in industrial and mobile applications. Digital hydraulics consist of fast switching valves arranged in parallel. Depending on the requirement, one or several valves are quickly  opened via an impulse code or pulse width modulation. This impulses control the flow in numerous tiny binary steps – thus the designation digital hydraulics. Fast switching valves are particularly robust and insensitive to contamination. As there are large delivery flows with every opening, they are de facto self-cleaning.


Tilting technology with axial piston pump, manifold with digital-hydraulic fast switching valves and control unit

Tilting technology shortens travel times

In addition to the use of stationary machines and systems, the scientists determined system advantages in the mobile use in trains with tilting technology. Safety regulations limit the admissible transverse acceleration of passenger trains to a maximum of 2 m/s². From that acceleration, objects start slipping off the tables and some muscular strength is necessary to remain seated upright. This limitation defines the maximum admissible curve speeds and thus the achievable travel times. Here, the tilting technology allows for considerable increase in the velocity while at the same time increasing the travel comfort for the passengers. Depending on the velocity, it tilts the bodies by up to eight degrees towards the curve inside and reduces the lateral acceleration. In this way, trains can go through curves up to 30 percent faster. Simultaneously, the tilting technology improves the passengers’ travel comfort also at slow velocities. For the technical implementation of the tilting technology, there have – until now – been pneumatic,electro-mechanical and conventional-hydraulic solution approaches.

Space-consuming electrical engineering

With the electro-mechanical solution, four electric motors with high-reduction ratio gears drive spindles which in each case vertically lift one carriage side. This solution is very dynamic and relatively easy to realize as regards the control engineering. On the other hand, gears and spindles are subject to heavy mechanical wear, especially as the application is characterized by contamination, vibrations and impacts. The four motor-gear assemblies require a lot installation space and must be directly placed at the relevant spindle. This considerably limits the flexibility of constructors and makes it almost impossible to compensate the failure of a drive unit by means of redundancies. If one of the components does not work properly, the trains must immediately reduce the velocity and go to a depot for repair. This again may considerably limit the availability.


View into the hydraulic tilting technology control center

Conventional hydraulics with high force density

The hydraulics off er very high force density, considerably compacter design than electro-mechanical drives and are extremely robust. Consequently, they are used in most concepts. A typical, conventional hydraulic solution for the tilting technology consists of four, single-acting cylinders installed at the four corners of every body. Via hydraulic quick-release couplings, they are connected to the hydraulic power unit with axial piston variable displacement pump, two redundant servo valves as well as six logic valves, placed centrally under every carriage.

The components are very compact and only need little installation space. Another advantage is the separate design of the hydraulics: Via oil lines, the power unit supplies the tilting cylinders from any position. This increases the flexibility for the design. Connection of the control technology has proven of value in numerous applications, the hydraulic control with servo and logic valves is, however, quite complex. Apart from that, the resulting energy requirement necessary to cool the hydraulic oil is higher. While the hydraulics tolerate vibrations and impacts almost without suff ering any wear, the contamination and low temperatures down to -40°C cause problems in this application which may even cause failures at the valves. That is why such system solutions often work with two redundant servo valves. If the active one fails during the journey, the redundant valve is, however, not able to take over automatically.

Digital hydraulics with system-typical advantages

The digital-hydraulic solution combines the advantages of the force density and the little installation space required for the hydraulics with a considerably increased availability and reduced power consumption. Such solution exemplary consists of an axial piston displacement pump in bent axis design, a manifold with seven fast switching, digital-hydraulic 4/3 seat valves and 3/2 seat valves for the neutral circulation and the emergency switch-off of the tilting system. Servo valves are completely omitted. The number of the logic valves can be reduced from six to one. The digital valves are controlled using a largeseries control unit. The control operates the digital valves using fast switching characteristics adjusted to the requirement, the characteristic curve of which has been adopted to the necessary switching times, system architecture and the error tolerance strategy.


Digital-hydraulic fast switching valves

If one digital valve fails, compensation is possible during the journey by means of the control and the other valves. The train can maintain the velocity provided for in the schedule and reaches its destiny in time. Replacing a defective digital valve is moreover much less expensive than the repair of an electromechanical or conventional hydraulic drive. The considerably extended maintenance intervals are an additional advantage. As there are large delivery flows with every opening, digital valves are de facto self-cleaning. An additional benefit of intelligent electronics: They internally analyze sensor data and record operating conditions. They transmit the latter to the train’s control system and in this way improve the system’s diagnostic capability.

Retrofit kit for Finish National Railway

The theoretical advantages that had first of all been determined by Tampere Technical University caused the Finish National Railway (VR) to intensively deal with the digital hydraulics topic. VR has operated the Pendolino tilting trains on the basis of the FIAT ETR 460 series since 1995 and has built up a fleet of 18 vehicles. The basic version has been adjusted to the special climatic conditions of Finland: arctic winters during which minus 30°C are not uncommon and warm summers with more than 25 degrees. The Pendolinos used in Finland with six carriages each are sensitively tilted in the curves by up to 8 degrees from a velocity of 80 km/h. Also at higher velocities, the tilting velocity limits the transverse acceleration to less than 0.65 m/s². In this way, the Pendolinos can drive through curves about 30 percent faster than conventional trains – a considerable time gain improving VR’s competitiveness over these distances.

The systematic analyses of the life cycle costs, however, showed maintenance costs for the servo valve solution that were far above-average. Particularly the extreme temperature diff erences and contamination of the hydraulic oil led to servo valve failures and thus, the complete tilting technology of the train had to be switched off . Consequently, the trains must considerably reduce the curve speeds immediately; afterwards, the servo valve will be replaced in the vehicle depot. Considered over a life cycle of 40 years, refitting with the more robust digital hydraulics with identical performance will thus pay out within a short period.

Scientific expertise for alternatives

The Finnish National Railway secured itself the expertise of Tampere Technical University in the field  f digital hydraulics for a joint project. By order of the National Railway, the scientists worked out a concept study. In the latter they proved that the Pendolinos can be refitted with a digital-hydraulic solution in an economic way. By this refitting, TUT expected clearly increased availability of the tilting technology and significantly reduced maintenance costs. The results of the study caused VR to develop a digital hydraulics retrofit kit in a project with  Bosch Rexroth under scientific supervision of TUT.

Plug & play solution reduces complexity

Right from the beginning, the focus was on two targets: Clear improvement of the availability on the one hand and little complexity and costs of the retrofitting on the other hand. With a quantity of more than one hundred necessary retrofit kits, one for every carriage of the trains, clear cost eff ects could be achieved here. VR specified the existing mechanical, hydraulic and electric interfaces as basic conditions. The specified solution must moreover be seamlessly integrated into the existing control technology of the trains. VR attached particular importance to reduction of the complexity by awarding the order to a system partner, Bosch Rexroth. With the retrofit kit, the drive and control manufacturer designed a plug & play solution from steel construction to digital hydraulics and the electronic control unit including the parameterized software.


Circuit diagram of the conventional, hydraulic solution



Circuit diagram of the digital-hydraulic solution

Reliability proven – energy efficiency improved

The retrofit kit was validated in several steps. After release of a prototype, two carriages of one train were equipped with the retrofit kit.  Measuring devices at all carriages provided the data for direct comparison of the original equipment with the digital hydraulic solution. The first tests were carried out and analyzed in the depot manually. The scientists rendered the proof that the new solution satisfies the required functionalities, i.e. the same tilting performance. Afterwards, the two prototypes had to prove their value in practice for a period of two years. During that time, they were exposed to Finland’s temperature range several times. Result: Within the two years, there was no single failure. Particularly the self-cleaning eff ect of the fast switching valves increases the resistance to contamination in the oil to the expected extent. Another reason for this is the simplified set-up. The retrofit kit is based on a manifold standardized for industrial applications.

Higher energy efficiency reduces operating costs

Consideration of the life cycle costs in addition to the acquisition costs and the maintenance expense also comprised the operating costs. Here, the digital-hydraulic retrofit kit scored by lower power consumption of the Pendolinos. It was shown by the measurements during the test operation that as opposed to the systems with servo valves, the retrofit kit did no longer need any oil cooling. Even on the hottest summer days, the oil temperature remained in the uncritical area. This resulted in clearly improved energy efficiency of the tilting technology.


The new, digital-hydraulic solution off ers the same tilting performance as the previous analog system with clearly improved availability and reduced maintenance costs. Additionally, it improves the energy efficiency. On the basis of the positive experience, the Finish National Railway will equip their entire Pendolino fleet with the retrofit kits over the next years. The retrofit kit satisfies all European railway safety provisions and is commercialized worldwide.

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